1. Field of the Invention
This invention relates to a sampling rate converter and, more particularly, is directed to an apparatus by which the sampling rate of a digital signal may be converted from an initial sampling frequency to a different sampling frequency having a low correlation therewith.
2. Description of the Prior Art
It has been proposed, for example, as disclosed in detail in Japanese Laid-Open Patent 60-4324, published Jan. 10, 1985, and having a common assignee herewith, to provide a sampling rate converter primarily intended for use in connection with digital audio signals which have a relatively low sampling rate, and by which the initial sampling rate can be converted to an arbitrarily different sampling frequency having low correlation with the initial sampling rate. In such proposed sampling rate converter, the input digital signal is oversampled and the resulting oversampled signal is interpolated and then applied to a digital low pass filter, whereupon, the output of the digital low pass filter is supplied to a decimating circuit for generating therefrom a digital signal having the desired sampling frequency. Although the foregoing sampling rate converter comprising an interpolation circuit, a digital low pass filter and a decimation circuit, connected in series in the order named, can effect conversion of the sampling rate without aliasing noise, the digital low pass filter connected between the interpolation circuit and the decimation circuit has to be of a very high degree or order, particularly if the input digital signal is a digital video signal. For example, if the previously proposed sampling rate converter is used for converting a component-type digital video signal having a sampling frequency of 13.5 MHz into a digital video signal of the PAL composite-type having a sampling frequency of 17.734 MHz, the digital low pass filter, which is desirably of the finite impulse response (FIR) type, would need to have 16,500 stages in order to provide the sampling rate converter with desired predetermined conversion characteristics for maintaining the Nyquist frequencies in the transmission channel. Such a large number of stages in the digital low pass filter is impractical in that it makes it difficult to design the low pass filter as an integrated circuit.